This invention relates to an adjuster arrangement for a drum brake that establishes a running clearance between a friction surface and a drum and prevents an over adjustment of the running clearance when an actuation force moves a web into engagement with a rotatable member during a brake application.
Non-servo drum brakes as disclosed in U.S. Pat. Nos. 4,101,011; 4,220,227 and 4,223,765 have an extendable strut located between a first web of a first brake shoe and a second web of a second brake shoe to control a running clearance between the brake shoes and a drum. A pawl located on the first brake shoe has an arm that engages a ratchet wheel on the extendable strut to provide for an extension of the extendable strut when the running clearance reaches a maximum width. This type of structure for adjusting the running clearance functions in an adequate manner in a drum brake during a normal braking application. However, it has been observed in a brake system that includes a traction control and/or a sway control system it is possible to over adjust the running clearance. On investigation, it was found that a cause of the over adjust is due to the level of the actuation force acting on the first and second webs to move the first and second brake shoes into engagement with the drum actually compressed the brake pads and/or distorted the structural components sufficiently to indicate a greater linear distance than actually needed to move from a position of rest to an actuation position. Thus on termination of the actuation force, when the running clearance is over adjusted, it is possible that the brake shoes in an extreme situation may actually remain engaged with the drum. In such an extreme situation, the brake shoes cause a drag that remains until wear has removed lining material from the brake shoes.
A primary object of the present invention is to provide a drum brake with an arrangement to prevent over adjustment of an extendable strut that controls running clearance.
According to this invention, the drum brake has a hydraulic actuator that engages a first web of a first brake shoe and a second web of a second brake shoe. An extendible member is located between the first web and the second web and is extendable to define a running clearance in the rest position between friction pads associated with the first and second brake shoes and the rotatable member. A lever arm is mounted on the first web and engages the extendable member. A resilient member extends between the lever to the second web to urge the first and second webs into engagement with the hydraulic actuator. When an operator desires to effect a brake application, pressurized hydraulic fluid is supplied as an input to the hydraulic actuator for developing an actuation force to move the first and second brake shoes from a rest position to a position where the corresponding friction pads thereon engage a rotatable member to effect the brake application. As the first web moves through the running clearance, the lever arm pivots with respect to the first web and when the running clearance reaches a predetermined maximum gap the lever pivots sufficiently to provide for extension of the extendible member. The extension of the extendible member moves the first and second shoes closer to the rotatable member and thereby establishes a minimum width for the running clearance between the friction pads and the rotatable member. The adjustment arrangement is characterized in that the first web has a first engagement surface connected to the hydraulic actuator and a second engagement surface connected to the extendible member. The first engagement surface receives the actuation force directly from the hydraulic actuator in moving the first brake shoe through the running clearance to the position where the friction pad engages the rotatable member during a brake application. The first web has a cantilever arm through which the first engagement surface is connected to the second engagement surface and as a result movement of the second engagement surface in response to the actuation force is limited as a function of the resiliency of the cantilever arm. However, a manual input force applied to the second engagement surface through the extendible member is directed into the first engagement surface without substantially flexing the cantilever arm to effect a manual brake application.
An advantage of this invention resides in a drum brake wherein a hydraulic actuation for developing an actuation force to move first and second webs associated with corresponding friction shoes into engagement with a rotatable member to effect a brake application is modified through a resilient member to limit an adjustment of the running clearance.